The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Digital to analog converters (DACs) convert a digital signal to an analog signal. Analog to digital converters (ADCs) convert an analog signal to a digital signal. For example only, some audio signals are stored in digital form (for example MP3s and CDs). The digital signals must be converted into analog signals in order to be heard through speakers. Video signals from a digital source must be converted to analog signals if they are to be displayed on an analog monitor.
Referring now to FIG. 1, a heterodyne modulating system 10 for generating a digitally-modulated carrier is shown. For example only, the system 10 may be used in a cable television system or another system. The system 10 includes a circuit 20 such as a field programmable gate array (FPGA), application specific integrated circuit (ASIC) or other circuit. Digital signals are output by the circuit 20 to a digital to analog converter (DAC) 24, which converts the digital signals to analog signals.
The analog signals output by the DAC 24 are input to an intermediate frequency (IF) filter 28, which may perform filtering such as bandpass filtering in a first frequency range. An output of the IF filter 28 is input to a mixer 32 that also receives a signal from a local oscillator (LO) 34. An output of the mixer 32 is input to an IF filter 38, which performs filtering such as bandpass filtering in a second frequency range. An output of the IF filter 38 is input to a mixer 40, which also receives a signal from a variable LO 42. An output of the mixer 40 is input to a radio frequency (RF) filter 46. An output of the RF filter 46 is amplified and output to a buffer/amplifier 48. An output of the buffer amplifier 48 generates the RF output signal at 49.
Digital modulation such as quadrature amplitude modulation (QAM) may be performed by the circuit 20 in the digital domain. The digital modulated carrier signal is converted to an analog carrier signal by the DAC 24. The IF filter 28 removes an image and other spurious content from the output of the DAC 24.
One or more frequency conversions are performed to convert the analog carrier frequency to a signal at a desired channel frequency. In FIG. 1, a low intermediate frequency (IF) signal produced by the DAC 24 is up-converted to a high IF signal. The high IF signal is filtered by the second IF filter 38. The high IF signal is then converted to an RF carrier signal by the mixer 40 and the variable LO 42. The output of the mixer 40 is filtered by the IF filter 46, which passes the entire cable band. The buffer/amplifier 48 provides the final RF output.
With the availability of high-speed, high-performance DACs, it is possible to directly produce the desired RF output signal. Referring now to FIG. 2, a direct modulating system 50 that directly produces the desired RF output signal is shown. In the direct RF approach, a DAC 58 receives the output of a circuit 54 such as a FPGA, ASIC or other circuit and produces the RF carrier signal directly at a final RF frequency. There is minimal filtering between the output of the DAC 58 and the RF output signal at 68. In FIG. 2, the output of the DAC 58 is filtered by an RF filter 62 and amplified by a buffer/amplifier 66. The buffer/amplifier 66 outputs the RF output signal at 68.
While having many advantages, the direct approach has increased exposure to DAC spurious outputs. In the system 10 of FIG. 1, the IF filters 28 and 38 limit bandwidth of the output signal of the DAC 24 and thus reduce a contribution of the DAC 24 to the overall spurious outputs of the system. In the system 50 of FIG. 2, there is minimal filtering between the DAC 24 and the RF output signal at 68. Any spurious signals at the output of the DAC 24 will be passed directly to a system receiving the RF output signal.